It is advantageous to employ a downhole pneumatic motor for drilling boreholes, especially when encountering rock formations which can be drilled more economically with air and gas techniques. In my previous patents, U.S. Pat. No. 4,432,423 and U.S. Pat. No. 4,434,862 there is disclosed a downhole pneumatic motor connected to the lower end of a non-rotating drill string which rotates a drill bit through a reduction gear system. Reference is made to my previously filed patents for further background of the present invention.
The downhole pneumatic motor is actuated by compressed air or gas which usually is supplied to the drill string via surface compressor systems. The downhole pneumatic motor converts the internal energy and kinetic energy of the air or gas flow under pressure to mechanical energy in the form of a rotating shaft. For the pneumatic motor to operate at a specific horsepower level, the air or gas flow through the motor and the pressure drop ratio across the motor must be maintained at a prescribed magnitude. In most air or gas pneumatic motor applications, the exhaust of the spent air or gas is to local atmospheric conditions. The atmospheric pressure conditions are not variable; therefore, by maintaining a constant injection pressure and flow rate to the motor, the horsepower output of the motor can be maintained. However, for a downhole pneumatic motor, the spent air or gas exits to the annulus at the bottom of the hole. The back pressure on the air or gas exiting the downhole pneumatic motor is partially the result of the friction losses in the flow stream as the air or gas moves up the annulus. Also, the spent air or gas is required to lift rock cuttings from the bottom of the hole. Because of these factors, the bottomhole pressure can vary depending upon the amount of rock cuttings that is in the annulus return air or gas flow, or whether formation water has entered the annulus and must also be carried to the surface by the return air or gas flow.
If the exit (for air or gas flow) from the downhole pneumatic motor were exposed to these variations in bottom hole pressure, the pressure drop ratio through the downhole pneumatic motor will vary and therefore vary the horsepower output of the motor itself. This would in turn affect the mechanical power to the drill bit and thereby the drilling efficiency.
This disclosure teaches method and apparatus by which the mass flow rate and pressure drop ratio through a downhole pneumatic motor are maintained constant as the bottom hole pressure varies, and therefore, the horsepower output of the downhole pneumatic motor is maintained constant.